In order to gain insight into the mechanisms responsible for in vivo platelet aggregation, thrombosis, and leucocyte adherence to microvessel walls, bioelectric potentials and current accompanying each of these intravascular phenomena will be measured in arterial and venous vessels of the living animal. Platelet aggregates and thrombi will be produced by: (1) passing electric current through the vessel wall, (2) laser irradiation of the blood flowing in the microvessel and (3) by iontophoretic deposition of ADP to the vessel wall. Enhancement of leucocyte adherence will be accomplished by laser irradiation of tissue adjacent to the microvessel under microscopic observation. In each case the intravascular events will be observed directly through the microscope and recorded on video tape for subsequent playback and analysis. Using microelectrodes, both extravascular and intravascular bioelectric measurements will be made and the magnitude, time course, and spacial characteristics of these compared with the dynamics of the intravascular events.